1. Field of the Invention
The present invention generally relates to a method for improving process window in semiconductor processes, and more particularly to a method for improving process window in semi-dense area by using phase shifter.
2. Description of the Prior Art
The need to maintain cost and performance competitiveness in the manufacture of semiconductor devices gives rise to integrated circuits with increasing density. As high integration is greatly required in manufacturing integrated circuits, the miniaturization of the circuit pattern formed on a semiconductor wafer, therefore, is consequently developed. Optical lithography is one of the techniques commonly employed to transform the miniaturized circuit pattern to define on the wafer.
Design rules define the space tolerance between devices or interconnect lines so as to ensure that the devices or lines do not interact with one another in any unwanted manner. One important layout design rule which tends to determine the overall size and density of the semiconductor device is the critical dimension (CD). A critical dimension of a circuit is commonly defined as the smallest width of a line or the smallest space between two lines. Another critical design rule defines the minimum of the width of a given feature plus the distance to the adjacent feature edge as the minimum pitch.
Once the layout of the circuit is created, the photolithographic process utilizes an exposure tool to irradiate a layer of photoresist on the wafer through a mask to transfer the pattern on the mask to the wafer. As the critical dimensions of the layout approach the resolution limit of the lithography equipment, proximity effects begin to influence the manner in which features on a mask transfer to the resist layer such that the masked and actual layout patterns begin to differ. Proximity effects are known to result from optical diffraction in the projection system. The diffraction causes adjacent features to interact with one another in such a way as to produce pattern-dependent variations; the closer together features are, the more proximity effect is seen.
One specific proximity effect related problem occurs when features are designed to have the same dimension, but are placed in a different proximity to other features in a layout. Features have edges in close proximity to other features (referred to as in dense area) that are more affected by proximity effects while features have edges that are relatively isolated are less affected by proximity effects. As a result, a feature in dense area tends to be printed differently than an isolated feature. For the manufacture of the semiconductor device in order to improve resolution and process window, the resolution enhanced technology (RET) has been applied, such as optical proximity correction (OPC), off axis illumination (OAI), and phase shift mask (PSM). Optical proximity correction is the process of selecting biasing of mask patterns to compensate for the pattern distortions occurring during wafer processing. By use of off off axis illumination technique, it is possible to separate the exposure in different directions.
A phase shift mask is essentially composed broadly of a light screen pattern and a phase shift pattern. The phase shift pattern plays the role of shifting an incident beam at a specific angle. Such a phase shift mask is designed to keep the amplitude of the light illuminated on the wafer constant at the exposing step and to minimize the proximity effect caused by interference between a beam passing through one line of the phase shift pattern and another beam passing through another line adjacent to the one line. Thereby, the resolution of the photosensitive film pattern is improved.
Scattering bars (also referred to as intensity leveling bars and assist features) have also been developed in order to minimize or eliminate proximity effects between xe2x80x9cisolatedxe2x80x9d and xe2x80x9cdensely packedxe2x80x9d edges of features in a lithographic process. Scattering bars are correction features (typically non-resolvable) that are placed next to isolated edges on a mask in order to adjust the edge intensity at the isolated edge to match the edge intensity at a densely packed edge. Thereby, by use of scattering bars causes the feature having at least one isolated edge to have nearly the same width as features having densely packed edges.
However, it does not take into consideration that inserting assist features into the spacing between features edges in semi-dense area is not feasible by use of state-of -the-art technology of manufacturing the photomask. Scattering bars are xe2x80x9csubresolutionxe2x80x9d reticle features because the corresponding image, when projected onto the resist layer, is below the resolution limit and does not substantially pattern the underlying resist layer. For the photolithographic technique known in the art, by use of scattering bars means to insert assist features whose widths are less than 0.07 xcexcm into the spacing between features edges in a semi-dense area of deep submicron. A process window, such as the depth of focus, exposure latitude, and the exposure energy window, plays a critical role in a photolithographic process. That is to say, by application of assist features, incorporated with off-axis illumination in order to increase the process window of deep submicron, encounters another problem that it is difficult to insert assist features into the semi-dense area when a photomask is manufactured, so that it""s hardly improve the process window in semi-dense area. What is needed is a more feasible method to increase the process window in a semi-dense area.
The present invention is directed to a method that satisfies this need to increase the process window in the semi-dense area during the manufacture of semiconductor devices. The method is provided for improving process window in semi-dense area by using a phase shifter. This method comprises a step of providing a transparent substrate. Then, at least two opaque regions are formed on the substrate. A phase shifter with a depth is formed in the substrate, wherein the phase shifter is formed in-between adjacent opaque regions. Thus, a phase shift mask with a light screen pattern is formed, which is designed to shift the phase of light in accordance with the thickness of the substrate by removing to a predetermined depth of the substrate corresponding to the space between opaque regions. As a result, the phase shifter shifts the incident beam at an angle that reduces the proximity effect and improves the optical contrast and the depth of focus (DOF) to get wider process window.
It is another object of this invention that a method for generating a phase shift mask to reduce the proximity effect is provided.
It is a further object of this invention that a method for improving the optical contrast in semi-dense area is provided.
It is another further object of this invention that a method for improving the depth of focus in semi-dense area is provided.
In one embodiment, a method for improving process window in semi-dense area by using a phase shifter is disclosed. The method comprises a step of providing a transparent substrate, which can be a quartz substrate. Then, at least two opaque regions are formed on the substrate, wherein each opaque region is a feature with a certain width of a photomask, and there is a distance between two adjacent features, which is defined as a space. Duty ratio is defined as the ratio of a space to a feature width. The step of forming the opaque regions comprises a step of forming an opaque layer on the substrate. Then, a patterned photoresist is formed on the opaque layer, wherein the patterned photoresist defines the opaque regions on the opaque layer. Then, the opaque layer is etched to form the opaque regions on the substrate by using the patterned photoresist as a mask. A phase shifter with a depth is formed in the substrate, wherein the phase shift is formed in between adjacent opaque regions. The step of forming the phase shifter comprises a step of forming a patterned resist mask on the substrate, wherein the patterned resist mask defines the phase shifter on the substrate. Then, a portion of the substrate is etched to form the phase shifter with a depth in the substrate using the resist mask, wherein the phase shifter is formed in-between the adjacent opaque regions. The etching depth of the substrate is depended on the angle needed to shift the incident beam that reduce the proximity effect. As a result, the phase shifter shifts the incident beam at an angle, such as about 60 degree, that decreases the proximity effect and improves the optical contrast and the depth of focus (DOF) to get wider process window.